Sphere Gap Breakdown Voltage Tables

The following two tables give the breakdown voltages for a standard sphere
gap of the specified sphere size and spacing. The standard accuracy is ±3% for
gaps less than half the sphere diameter and 5% for gaps larger than that. The
first table is for a gap with one terminal grounded, typically a vertical gap.
There are two columns for each sphere size depending on whether it is a positive
or negative impulse. The second table is for a gap with both terminals at HV. The
values from both of these tables will need to be corrected for current atmospheric
conditions, particularly humidity. More details on sphere
gap construction and procedures is available.

Sphere gap with one terminal grounded

As the field gets more uniform (i.e. the gap is a smaller fraction of the sphere
diameter), the difference in voltage for the two polarities becomes less. In
general, voltages for spheres larger than the largest for which there is a voltage
will be the same. That is, 200 cm spheres with a 0.5 cm gap will breakdown at
the same voltage as a 15 cm sphere at the same spacing.

Sphere Diameter (cm)

Gap Spacing

5

10

15

25

50

100

150

200

cm

A

B

A

B

A

B

A

B

A

B

A

B

A

B

A

B

0.5

17.4

16.9

16.8

16.9

1.0

32.0

31.7

31.4

31.2

31.4

1.5

44.7

45.5

44.7

45.1

44.7

45.1

44.7

2.0

57.5

58.0

58.0

58.0

58.0

2.5

71.5

71.5

71.5

71.5

3.0

85

85

85

85

3.5

95.5

96

97

97

97

4.0

106

108

108

110

110

110

5.0

123

127

127

132

135

136

136

7.5

181

187

195

196

199

199

10.0

257

268

259

259

262

262

262

12.5

277

294

315

317

15.0

309

331

367

974

383

384

384

384

17.5

336

362

413

425

20.0

452

472

500

500

500

25.0

520

545

605

610

30.0

575

610

700

715

730

735

735

740

35.0

725

755

785

800

40.0

862

885

940

950

960

965

45.0

925

965

50.0

1000

1020

1110

1130

1160

1170

75.0

1210

1260

1420

1460

1510

1590

100.0

1870

1900

Symmetrical sphere gaps

This data applies to gaps where neither sphere is grounded, or, where the gap
is so far from the surroundings that the fields relative to ground have no influence.